For some time, aluminum seam-free can bodies have been produced by cold forming methods including drawing steps, which cans when lined have been used for holding beverages and food products. In the latter use, the cans are referred to as sanitary containers. More recently seam-free can bodies have been formed from tinplated steel, the tinplated steel can bodies being stronger and cheaper to produce than the equivalent aluminum products. Although the tinplated steel can bodies thus produced have been satisfactory for containing beverages, unless they are lined (enameled) their corrosion resistance has not been great enough to give satisfactory service as plain sanitary containers. This is despite the fact that the tinplate starting stock for such seam-free tin can bodies does have satisfactory corrosion resistance, the cold forming of the steel can body, whether by draw and redraw or by draw and wall ironing steps, quite evidently destroyed this original satisfactory corrosion resistance of the electrotinplated starting stock.
Applicants have discovered that by treating the cold formed cups making up the bodies of the tinplated steel containers at elevated temperatures near but below the melting point of tin for controlled time periods, the satisfactory corrosion resistance of the initial tinplate starting stock can not only be restored but can even be improved.
It has been proposed to continuously heat treat tinplated continuous strip following a continuous flow brightening step to improve the shelf life of cans manufactured from the heat treated tinplate (U.S. Pat. No. 3,129,150 in which one of the applicants in the present case is a joint patentee). However, the end product of the patented process, at best, corresponds to the starting stock in the present invention and there was no reason to believe that a further similar heat treatment would improve the corrosion resistance of cold formed products.